M. D. Bender, G. B. Olson, Northwestern University, Evanston, IL
3-D local electrode atom probe (LEAP) tomography is utilized to support the computational design of nanodispersion-strengthened TiNi-based shape-memory alloys. A superelastic biomedical TixZr1-xNiyPt1-yAl alloy with improved fatigue resistance, strength, and radiopacity is designed for the application of stenting a superficial femoral artery. A dual-beam focused-ion beam supports the fabrication of Pt-containing FIM-specimens as the standard electropolishing technique is not sufficient for Pt-containing alloys. Using LEAP tomography, the otherwise unattainable B2-L21 phase relations for this class of alloys at 600°C are mapped. Results show that an unintended effect of Zr and Pt is to stabilize the undesirable metastable Ni4Ti3 phase; therefore, phase relations of Ni-rich precipitates are also studied and optimized alloy compositions are designed to avoid them. Models are developed to describe solid solution strengthening, precipitation strengthening, and transformation Af temperatures. Mass attenuation is taken as a quantitative metric of alloy radiopacity. Precipitate growth and coarsening was investigated with tomography, revealing that optimal microstructures occur in the early stages of precipitation rather than at equilibrium, so precipitate capillarity energy is also considered. Mechanical testing is used to validate the expected high-performance properties of designed alloys. All results are used to build a more robust Thermo-Calc thermodynamic database enabling other TixZr1-xNiyPt1-yAl designs for alternative applications.
Summary: In support of computational design, 3D local electrode atom-probe (LEAP) microanalysis is applied to quantitative study of nanoscale L21 aluminide precipitation strengthening as a mechanism for increasing the yield strength of transformable TiNi-based, Zr- and Pt-containing shape-memory alloys to increase fatigue resistance by reducing accomodation slip.
